A conventional video conferencing end-point includes a codec, a video display, a loudspeaker and a microphone, integrated in a chassis or a rack. In larger end-points for use in meeting and boardrooms, the audio equipment is installed separately. The microphone is often placed on the meeting table so as to bring the audio recorder closer to the audio source.
However, personal video conferencing end-points, often referred to as desktop systems, are now becoming more common in offices as a substitute or supplement to larger end-points or to traditional telephony. Personal equipment is more portable, and is likely to be placed close to the user on a table. Thus, all the equipment belonging to one end-point, including the microphone is integrated in one device.
Microphones for desktop systems are normally placed where practically feasible, and fully integrated into the system assembly. In conventional desktop systems, the microphone is therefore often positioned the enclosure of the desktop, at a certain height above the tabletop. This implies several audio problems, which will be discussed in the following.
In nearly all in-house environments, degradation of audio quality appears due to reflections caused by interior, walls, floor and ceiling. In audio captured by a microphone in a conventional desktop system, this is a considerable problem because the tabletop will cause a strong reflected audio signal from the audio source contributing to the direct signal with a relatively short delay. The situation is illustrated in FIG. 1. Reflections that reach the microphone after the direct sound cause a phenomenon known as comb filtering. The appearance of a single reflection in a frequency response looks similar to the teeth in a hair comb. Comb filtering due to a single 2 ms reflection is illustrated in FIG. 2.
The upper chart shows the resulting impulse response from an audio source to the microphone when only the direct path and a single 2 ms reflection path are considered. The lower chart of FIG. 2 shows the corresponding impulse response in the frequency domain. As can be seen, the comb filtering nulls in the frequency response due to a 2 ms reflection will be spaced 1/0.002=500 Hz. The first null will appear at 500/2=250 Hz. The nulls in the frequency response attenuate certain frequencies, and degrade sound quality.
By placing the microphone closer to the tabletop, the frequency of the first null as well as the spacing between the additional nulls will be increased.
In high quality desktops, full audible bandwidth may often be required. To avoid undesirable nulls in the transmitted bandwidth, the microphone must be positioned closer to the tabletop than is achievable by positioning the microphone in the enclosure of the desktop unit. It is known from prior art that placing the microphone as close to the reflecting surface as possible without touching it, can reduce the effect of the reflection because the reflected signal and the direct signal will merge into each other as the distance between the surface and the microphone approaches zero. This is utilized in external table microphones, which are commonly connected to larger conferencing end-points.
In desktop systems however, the microphone should be fully integrated, but this implies several problems related to installation near the tabletop in a controlled and mechanically robust way and still achieve the audible benefits.
One problem is that a microphone placed at the underside of the desktop system is exposed to mechanical damages, and a microphone is particularly sensitive to this.
Further, if the desktop system contains loudspeakers used for two-way communication, there is a strong possibility for transmission of structure borne sound and vibrations excited by the speakers to the microphone. Such vibrations will also reduce the quality of sound picked up by the microphone, and they may be disturbing for the acoustic echo control.
Further, a fully integrated microphone solution is specific to the system design, and cannot easily be used as a module in a new or different system.
The requirements for sound quality are increasing as sound pickup is made using higher bandwidth audio. Also, desktop systems are often used for two-way communication, making acoustic echo and feedback control an important issue.
Microphone design, placement and assembly are therefore critical factors for the optimization of sound quality.